Final answer:
Erik Erikson developed the eight stages of life in his psychosocial theory, focusing on lifelong personality development influenced by social interactions.
Step-by-step explanation:
The psychologist who developed the eight stages of life in his psychosocial theory was Erik Erikson. His theory was a departure from Sigmund Freud's psychosexual stages, proposing that personality development occurs throughout one's lifespan and is more socially influenced. Erikson believed that the development of ego identity, or our sense of self, is influenced by our social interactions. Erikson's psychosocial theory emphasizes that at each stage, we face a conflict or task, and our ability to resolve these issues largely impacts our personality development and sense of competence.